1. Technical Field
The present invention relates to an optical communications system suitable for carrying broadband communications traffic.
2. Description of Related Art
Optical communications networks potentially offer very high capacities. However, in practice it has proved difficult to design appropriate switching technologies to realise the full potential of optical communications networks. Opto-electronic switching designs limit the bit-rate that can be handled by the network. Control and management of the switching function also provides further difficulties and there is a need to minimise software costs for call processing, network intelligence, network management and service management.
The present applicant""s earlier international patent application WO95/26592, to which the reader is directed for relevant technical background, describes and claims an optical communications system in which a number of terminals are interconnected via passive optical networks (PONs) and by a wavelength-dependent router at the hub of the network. Each terminal includes means, such as a tuneable transmitter and receiver, that allows it to select one of a number of different wavelength channels for transmission and reception in different time slots. In this way, the switching function is distributed to the edges of the network, while the core of the network functions using chiefly or entirely passive routing devices. This overcomes many of the problems discussed above. However, there is a problem in that distributing the switching function to the edges of the network potentially imposes a large signalling overhead for the transmission of network control information such as bandwidth or packet requests and a resource allocation map indicating the wavelength channels and time slots allocated to each request.
According to a first aspect of the present invention, there is provided a method of operating an optical communications system comprising a wavelength-dependent optical router-, a plurality of terminals, each terminal including at least one tuneable optical receiver, and at least one passive optical network interconnecting the plurality of terminals via the wavelength-dependent optical router, the method including:
(a) transmitting to each terminal in a first signalling phase, the identity of a wavelength channel assigned to the terminal, where some of the terminals are assigned a common wavelength channel;
(b) in a second signalling phase subsequent to the first signalling phase, transmitting network control signals to each terminal of the respective wavelength channel identified to the terminal in the first signalling phase; and
(c) in the data transmission phase subsequent to the first and second signalling phases, communicating data between terminals via the passive optical network and the wavelength-dependent optical router, where each terminal outputting data onto the passive optical network transmits on a respective wavelength channel selected depending on the control data received at the respective terminal in the second signalling phase.
The present invention provides a method of communicating control signalling to the edge of the network that makes highly efficient use of the network resources, thereby enabling all signalling messages transmitted from the network controller to the terminals to be received at each terminal by means of the same tuneable receiver as is used for data reception. In preferred implementations, a complete resource allocated map may be distributed to the terminals, which allows complete flexibility for the network controller to allocate time-slots and wavelengths and hence to provide faster responses to resource requests. This is achieved by using a two phase signalling process at the beginning of a data frame. The first phase, termed the meta-signalling phase, is used to assign a number of terminals to a given wavelength channel for the subsequent signalling phase. In the subsequent signalling phase control data, such as a complete wavelength/terminal allocation map, is transmitted to the terminals. The use of a two-phase process for signalling gives flexibility in the allocation of signalling wavelength channels.
According to a second aspect, the invention provides an optical communications system comprising a wavelength-dependent optical router, a plurality of terminals, each terminal including at least one tuneable optical receiver, and at least one passive optical network interconnecting the plurality of terminals via the wavelength-dependent optical router, the system being configured to operate according to a method which includes the following steps:
(a) transmitting to each terminal in a first signalling phase, the identity of a wavelength channel assigned to the terminal, where some of the terminals are assigned a common wavelength channel;
(b) in a second signalling phase subsequent to the first signalling phase, transmitting network control signals to each terminal of the respective wavelength channel identified to the terminal in the first signalling phase; and
(c) in the data transmission phase subsequent to the first and second signalling phases, communicating data between terminals via the passive optical network and the wavelength-dependent optical router, where each terminal outputting data onto the passive optical network transmits on a respective wavelength channel and data time-slot selected depending on the control data received at the respective terminal in the second signalling phase.
The invention also encompasses a network controller or a terminal arranged to operate in accordance with the method of the first aspect, and a communications system including such a controller and terminal.